1. Field of the Invention
The present invention relates to a cathode active material powder for a lithium secondary battery, which has a large volume capacity density, high safety, excellent durability for charge and discharge cycles, high rate discharge properties and low temperature characteristics.
2. Discussion of Background
Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Among them, a lithium secondary battery using a lithium-cobalt composite oxide (LiCoO2) as a cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can operate at a high voltage at a level of 4 V, whereby it has been widely used as a battery having a high energy density.
However, LiCoO2 contains scarce cobalt and is thereby expensive, and in the case of the non-aqueous type secondary battery using it as a cathode active material, there has been a problem of deterioration of the cyclic properties, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature. Further, it has been difficult to put a medium-size or large-size non-aqueous type secondary battery employing LiCoO2 into practical use, because of problems in safety particularly heat stability.
On the other hand, as a cathode active material available at a low cost and having high safety, a lithium-manganese composite oxide represented by LiMn2O4 has been studied, but is significantly inferior in the durability for charge and discharge cycles and the volume energy density to LiCoO2 and is thereby hardly used practically.
In order to solve such problems, JP-A-2002-100357 by the present inventors proposes a mixture of a lithium-nickel-manganese-M composite oxide represented by LixNiyMnl-y-zMzO2 (wherein 0.9≦x≦1.2, 0.40≦y≦0.60, 0≦z≦0.2, and M is selected from Fe, Co, Cr and Al atoms) and a lithium-cobalt composite oxide having a R-3m rhombohedral structure and represented by LixCoO2 (wherein 0.9≦x≦1.1) However, although such a cathode active material has improved safety and has high rate charge and discharge properties which are by no means inferior, but one which sufficiently satisfies the volume capacity density has not yet been obtained.
Further, JP-A-2002-100358 proposes a lithium secondary battery provided with a cathode active material layer containing a lithium transition metal composite oxide as the main component, wherein the lithium transition metal composite oxide is a mixture of a lithium-nickel-manganese-M composite oxide represented by LixNiyMnl-y-zMzO2 (wherein 0.9≦x≦1.2, 0.40≦y≦0.60, 0≦z≦0.2 and M is selected from Fe+, Co, Cr and Al atoms) and a lithium-manganese spinel composite oxide having a Fd3m spinel structure and represented by LipMn2O4 (wherein 1≦p≦1.3). However, with such a cathode active material, durability for charge and discharge cycles is improved, but the volume capacity density is unsatisfactory.
Still further, in order to improve the weight capacity density and the cyclic charge-discharge properties of the positive electrode, JP-A-2000-82466 proposes a cathode active material wherein the average particle size of lithium composite oxide particles is from 0.1 to 50 μm, and at least two peaks are present in the particle size distribution. Further, it has been proposed to mix two types of cathode active materials having different average particle sizes to prepare a cathode active material wherein at least two peaks are present in the particle size distribution. In such a proposal, there may be a case where the weight capacity density and the cyclic charge-discharge properties of the positive electrode can be improved, but one satisfying all of the volume capacity density, the safety, the cyclic properties and the weight capacity density of the positive electrode, has not yet been obtained.
Further, JP-A-2001-80920 proposes an agglomerated lithium composite oxide having a compression breaking strength per grain of from 0.1 to 1.0 gf, which is represented by the formula LixNil-y-zCoyMezO2 (wherein Me is a metal atom with an atomic number of at least 11 or a transition metal element other than Ni and Co, 0≦x≦1.1, 0≦y≦0.6, and 0≦z≦0.6) and which is an agglomerated lithium composite oxide formed by agglomeration of a fine powder. However, such a composite oxide, which contains nickel, has a problem that the safety is poor, and the high rate discharge properties are inferior. Besides, with such a small range of compression breaking strength, it is not possible to obtain a lithium composite oxide having adequately satisfactory properties with respect to e.g. the volume capacity density, the safety, the cyclic properties and the high rate discharge properties.
As described above, a lithium secondary battery employing a lithium composite oxide as a cathode active material which fully satisfies the volume capacity density, the safety, the cyclic properties, the high rate discharge properties, etc., has not yet been obtained by prior art.